equivalent effect to https://github.com/plougher/squashfs-tools/pull/69
but simpler

Index: squashfs-tools/mksquashfs.c
--- squashfs-tools/mksquashfs.c.orig
+++ squashfs-tools/mksquashfs.c
@@ -35,7 +35,10 @@
 #include <stddef.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#ifdef linux
+#include <sys/sysinfo.h>
 #include <sys/sysmacros.h>
+#endif
 #include <fcntl.h>
 #include <errno.h>
 #include <dirent.h>
@@ -50,7 +53,6 @@
 #include <sys/wait.h>
 #include <limits.h>
 #include <ctype.h>
-#include <sys/sysinfo.h>
 
 #ifndef linux
 #define __BYTE_ORDER BYTE_ORDER
@@ -4348,6 +4350,7 @@ void initialise_threads(int readq, int fragq, int bwri
 	sigemptyset(&sigmask);
 	sigaddset(&sigmask, SIGQUIT);
 	sigaddset(&sigmask, SIGHUP);
+	sigaddset(&sigmask, SIGALRM);
 	if(pthread_sigmask(SIG_BLOCK, &sigmask, NULL) != 0)
 		BAD_ERROR("Failed to set signal mask in intialise_threads\n");
 
@@ -4363,7 +4366,9 @@ void initialise_threads(int readq, int fragq, int bwri
 		BAD_ERROR("Failed to set signal mask in intialise_threads\n");
 
 	if(processors == -1) {
-#ifndef linux
+#ifdef _SC_NPROCESSORS_ONLN
+		processors = sysconf(_SC_NPROCESSORS_ONLN);
+#else
 		int mib[2];
 		size_t len = sizeof(processors);
 
@@ -4380,8 +4385,6 @@ void initialise_threads(int readq, int fragq, int bwri
 			ERROR_EXIT("  Defaulting to 1\n");
 			processors = 1;
 		}
-#else
-		processors = sysconf(_SC_NPROCESSORS_ONLN);
 #endif
 	}
 
@@ -5195,6 +5198,7 @@ int get_physical_memory()
 	long long page_size = sysconf(_SC_PAGESIZE);
 	int phys_mem;
 
+#ifdef linux
 	if(num_pages == -1 || page_size == -1) {
 		struct sysinfo sys;
 		int res = sysinfo(&sys);
@@ -5205,6 +5209,7 @@ int get_physical_memory()
 		num_pages = sys.totalram;
 		page_size = sys.mem_unit;
 	}
+#endif
 
 	phys_mem = num_pages * page_size >> 20;